Techniques are known for forming opto-electronic devices such as light-emitting diodes (LEDs) over a substrate. Supported on the substrate, each such device comprises a layer of electroluminescent material disposed between two electrodes referred to as the anode and cathode. The device may also comprise a charge injection layer and/or a charge transport layer disposed between the electroluminescent layer and one (or both) of the electrodes. In modern devices, the electroluminescent layer, charge injection layer and/or charge transport layer may be formed of an organic polymer. An LED comprising one or more such organic layers may be referred to as an organic LED (OLED). In a particular application, an array of these devices may be formed over a substrate such as glass or transparent plastic in order to produce an electronic display screen for a computer, television, mobile terminal or other appliance.
Referring to FIG. 1, the method of forming a traditional OLED begins by providing a substrate 102 and then forming an anode 104 over the substrate 102.
The anode 104 is typically formed of a transparent metal-based material such as indium tin oxide (ITO) or other transparent conducting oxide (TCO). A hole injection layer 106 is then formed over the anode 104, a hole transport layer 108 is formed over the hole injection layer, an electroluminescent layer 110 is formed over the hole transport layer 108, and a cathode 112 is formed over the electroluminescent layer 110. Each of the hole injection layer 106, hole transport layer 108 and/or electroluminescent layer 110 may be formed of an organic polymer. Advantages of organic polymers along with some suitable solutions and deposition techniques are discussed in international patent application publication no. WO 2006/123167. One particular example of a hole-injecting material is PEDOT: PSS. That is, an active component of polyethylene-dioxythiophene (“PEDOT” or sometimes just “PEDT”) doped with a matrix component of polystyrene sulfonate (“PSS”). The electroluminescent layer 110 itself may comprise an organic non-polymeric material, or a polymeric semiconductor, an example of which is poly(p-phenylenevinylene (“PPV”). Suitable deposition techniques for these layers such as spin coating, dip coating and printing will be familiar to a person skilled in the art.
In operation, when a potential difference is applied between the anode 104 and cathode 112, holes (h+) are injected into the device from the anode 104 and electrons (e−) are injected from the cathode 112. The holes and electrons combine in the electroluminescent layer 110 to form an exciton which then decays to emit light (visible to the user through the transparent polymer layers 108, 106 and the transparent anode 104 and substrate 102). Injection of the holes from the anode 104 is assisted by the hole injection layer 106, and transport of the holes from the anode 104 to the electroluminescent layer 110 is assisted by the hole transport layer 108.
As shown in FIG. 2, more recently a type of OLED has been developed in which a high-conductivity organic polymer 106′ is used to form the hole injection layer, allowing it to provide a function of both an anode and a hole injection layer.
That is to say, the hole injection layer 106′ acts as both a donor of holes and a means to aid the injection of holes into the electroluminescent layer 110. This means no separate blanket anode layer 104 is required, and hence the hole injection layer 106′ is formed directly over the substrate 102 without an intervening anode layer 104. This means no ITO or other transparent conducting oxides are required for the anode 104, which is advantageous because such materials are in limited supply and therefore costly, and can also be brittle. However, to reduce the sheet resistivity for large area devices, conducting metal tracks 103 (e.g. silver) are still required to be formed over the substrate 102 prior to or after deposition of the conducting polymer layer 106′. This metal anode tracking 103 makes electrical contact with the dual-purpose hole injection layer 106′ so that electrical signals can be applied to operate the device.
Such an arrangement is disclosed for example in: [1] “Large Area ITO-free Flexible White OLEDs with Orgacon™ PEDOT:PSS and printed Metal Shunting Lines”, Stephen Harkema et al, Hoist Centre, Organic Light Emitting Materials and Devices XIII edited by Frank So, Proc. of SPIE Vol. 7451 (2009), CCC code 0277-786X/09, doi: 10.1117/12.825246.
Another example is disclosed in: [2] “Highly efficient OLEDs on ITO-Free Polymeric Substances”, Karsten Fehse et al, Institut für Angewandte Photophysik, Organic Optoelectronics and Photonics, edited by Paul L. Heremans et al, Proc. of SPIE Vol. 6192 (2006), CCC code 0277-786X/06, doi: 1117/12.662936. In this paper the function of anode, hole-injection layer and hole transport layer are all combined into a single organic conducting polymer layer.
It has been observed that the sharp edges to the anode metal tracking 103 in such devices is liable to cause thinning of subsequently deposited active layers, resulting in non-uniform light output and/or early failure of the device due to electrical shorts between the anode tracking 103 and the electroluminescent layer 110.
One solution which can mitigate this problem to some extent for small area devices is shown schematically in FIG. 3. Here, a bank or passivation structure (an insulating layer) 105 is formed over the anode metal tracking 103 and substrate 102. An example is the patterned resist passivation layer found on top of the tracking in an Orbeos lighting tile (OSRAM). However, this is impractical for large area devices (over about 15-20 cm across) where conductive tracking 103 is required throughout the entire active area to achieve the required conduction.
Another solution might be to use a thicker layer of the organic conducting polymer 106′ to provide better coverage over the contact 103. However, this would have the disadvantage that it may compromise the optical properties of the device, resulting in a less efficient device because higher drive voltage conditions would be necessary to achieve comparable output to hole injection layer structures 106′ having more optimal thinness.
The Holst Centre has reportedly overcome the thinning problem by covering the anode metal tracking 103 with an insulating planarization layer and then etching this layer back to reveal the anode metal tracking structure with infill planarization (see reference [1] above). This technique has its disadvantages by requiring very tight control of the etch back process to achieve planarization, and possible degradation of the anode metal tracking due to oxidation and physical damage.
It would therefore be desirable to find an alternative solution to the problem of thinning of the hole injection layer over a conducting contact in a light-emitting device such as an OLED.